This invention relates to ground current arcing detection in large generator machinery, and more particularly to such arcing detection at the main bearings supporting the rotor shaft in the generator.
The rotor shaft in a large turbine generator and the two end bearings form a loop which acts as a secondary winding on a transformer. Voltage is induced in the rotor shaft due to flux density differences existing between the various flux paths within the machine. The flux density differences occur because the reluctance of the different flux paths within the machine is different. Consequently, a voltage is induced in the rotor shaft as it rotates through 180 electrical degrees. The end bearings are insulated from the machine frame. Without the end bearing insulation a current will flow from the rotor shaft to the machine frame due to the induced voltage in the shaft. Such currents do occur if the end bearing insulation breaks down. There is a film of lubricant between the bearings and the shaft in which ground current arcing takes place in such an instance. The current arcing operates to pit the bearing surfaces and cause eventual failure of the shaft and associated bearings. In the past it has been common to attempt to monitor the integrity of the bearing insulation by connecting a volt meter to the generator shaft by way of a brush and to measure the voltage drop across the oil film. In theory an increase in the level of this voltage indicates current arcing due to failure of the bearing insulation. However, arcing currents give rise to high frequency fluctuation in voltage signals which are not detected by most volt meters. As a consequence, ground current arcing can be occurring in the gap between rotor shaft and the bearing races, causing surface pitting, and remain undetected until bearing failure.
A method and apparatus for detecting ground current arcing in rotor bearings used on large generator machinery is needed to detect conditions indicating potential rotating machinery failures, and to pinpoint the specific areas within the machine at which the failure potential exists.